Thermal conversion is a process in which, by the application of heat, large hydrocarbon molecules are broken into smaller molecules with a lower boiling point. These operations are carried out in the industry of crude oil refining by plants such as a visbreaker, coker, and hydrocracker for obtaining intermediate or light cuts of higher value, from heavy residues of lower commercial value. The thermal cracking applied in the visbreaker process will also reduce the viscosity and pour point of the heavy residues.
It is well known that the fouling potential of a fluid can be estimated and characterized by the concentration of the dispersed phase, particularly by the concentration of the dispersed phase present in a specific size range. In hydrocarbon systems in particular, it has been recognized that the concentration of asphaltenes (i.e., carbon particles or opaque species) with linear dimension greater than about 2 microns in visbroken tars is a good indication of the fouling potential of the material.
The VSB process was developed some years ago with the intention of obtaining a viscosity decrease in heavy products in order to reduce the amount of higher valued flux to meet the viscosity specification of the finished heavy fuel product. Today, however, it is managed with substantially different objects, namely with the aim of obtaining a maximum transformation into middle and light distillates to meet the market requirements.
The controlling factor in obtaining a high conversion is the need to obtain a stable residue. In fact an increase of the cracking temperature certainly would involve a higher conversion in light and middle distillates, but it would produce a much more instable tar which would produce a final product outside the required stability specifications.
An increase of the light streams is achieved by increasing the cracking severity through an increase of the outlet furnace temperature of the Visbreaker furnace. While increasing this temperature arbitrarily will serve to drive the conversion rate higher, it also comes at the cost of producing a highly unstable tar as a precipitate in the process, with a high concentration of asphaltene particulates. This particulate matter constitutes a severe fouling threat to the energy recovery devices (i.e. furnace and heat exchangers) in the process. As such, in order to maximize the profitability a Visbreaker unit, it is desirable to optimize the outlet furnace temperature while maintaining the stability of the produced tar. While it is known that high temperature dispersants and anti-foulants can be introduced into the system to reduce the tendency and rate of fouling, prior art systems have not been entirely satisfactory in providing an automated system for determining an optimum type and/or quantity of chemical dispersants and anti-foulants to be introduced into the visbreaker unit in order to maximize plant profitability. The present teachings will show that if the fouling potential of the tar can be quantified, then the precise level of chemical inhibitor can be dosed to maximize the plant profitability.
Therefore, in one aspect the present invention provides a simplified, automated system and method that can easily be used to carry out optical analysis of visbroken tars and other fluid samples in order to characterize and quantify the concentration of particles within the fluid sample with high accuracy and reproducibility. In another aspect, the present invention utilizes these concentration measurements to determine the fouling potential of the visbroken tars, and regulates the introduction of chemical inhibitors into the visbreaker unit to improve the yield of light streams. In yet another or further aspect, a sequence of aliquots are prepared from the same sample at different dilutions to drive phase separation, producing a sequence of concentration measurements correlated to a classical measurement of peptization value (PV), a qualitative measure of the product quality. These and other aspects of the present invention will become apparent to those skilled in the art upon review of the following disclosure.